The invention relates to a control arrangement for a hydropneumatic suspension system, in particular for vehicles, the control arrangement having a pressure supply connection for connecting to a pressure supply, a return connection, a piston chamber connection adapted to be connected to a piston chamber of a suspension cylinder of the hydropneumatic suspension system and adapted to be acted upon by a piston chamber pressure, an annular chamber connection adapted to be connected to the annular chamber of the suspension cylinder and adapted to be acted upon by an annular chamber pressure, and at least one controllable valve arrangement which comprises a plurality of switch positions through which the pressure supply connection and the return connection are connectable to the piston chamber connection and the annular chamber connection, the annular chamber connection being in flow connection with the return connection via a first pressure-limiting line, and a hydraulically controllable pressure-limiting element being provided in the first pressure-limiting line, said pressure-limiting element comprising a control input adapted to be acted upon via a control line by a control pressure that corresponds to the piston chamber pressure.
Moreover, the invention relates to a hydropneumatic suspension system comprising such a control arrangement.
Hydropneumatic suspension systems are in particular used in vehicles with varying load conditions, for example in tractors having holding means for attachments. At least one suspension cylinder is disposed between sprung and unsprung masses of the vehicle. Said cylinder comprises a piston chamber that is connected to a first hydraulic accumulator and can be acted upon by pressurized hydraulic fluid, and that carries the suspension load. The at least one suspension cylinder often comprises in addition an annular chamber that sealingly surrounds the piston rod of the suspension cylinder.
With increasing suspension load, hydraulic fluid is pushed out of the piston chamber into the first hydraulic accumulator so that the relative position of the sprung and unsprung masses changes. The change in the relative position is detected by sensors that are connected to an electrical control unit. The electrical control unit controls at least one valve arrangement such that the valve arrangement changes its switch position and connects the piston chamber to the pressure supply connection so that hydraulic fluid is fed to the piston chamber until a predefinable relative position is reached again. With decreasing suspension load, hydraulic fluid flows out of the first hydraulic accumulator into the piston chamber so that the relative position of the sprung and unsprung masses changes again. The change in the relative position is detected again so that the valve arrangement changes again its switch position and connects the piston chamber to the return connection. Then, hydraulic fluid can flow out of the piston chamber until the predefined level is established again.
Such suspension systems are in particular used in front axle suspensions of agricultural tractors. During work using a mounted tractor plow, the rear axle of the tractor is greatly loaded and the front axle is greatly relieved. Reversed conditions occur during work using a front loader. Thus, agricultural tractors are operated under very different operating conditions with very different front axle loads. It is known to those skilled in the art that hydropneumatic suspension systems have the property to stiffen disproportionately with increasing load. This is not always of benefit to the suspension properties. Thus, it is often attempted to counteract this property by preloading the suspension. In addition, preloading has the advantage that the operating range of the hydraulic accumulator is not exceeded even in the case of a greatly varying suspension load.
For preloading, in addition to the piston chamber, the annular chamber also of the at least one suspension cylinder is often acted upon by pressurized hydraulic fluid, the annular chamber being connected to a second hydraulic accumulator. In this connection, suspension systems are known in which the annular chamber is acted upon by a constant pressure and, as a result, the suspension is subjected to a constant preload. However, it is often useful, in particular in the case of low front axle loads, to increase the preload, thereby hardening the suspension. The reason for this is that low front axle loads of tractors are caused by heavy attachments at the rear, for example by a heavy mounted tractor plow, which also greatly increase the moment of inertia about the transverse axis of the tractor. This results in strong oscillations about the transverse axis of the tractor when driving over uneven ground. These oscillations are usually designated as “pitching oscillations”. These pitching oscillations can cause the suspension to bottom out. In the case of a constant preload, the latter has to be selected high enough that bottoming out cannot occur, not even under low axle loads. However, this has the disadvantage that the suspension of an unloaded tractor is very hard and therefore offers only limited driving comfort.
Suspension systems having a variable annular chamber pressure are also known. They avoid the aforementioned disadvantage by setting a high annular chamber pressure for the tractor with a heavy rear-mount attachment and a low front axle load and thus harden the suspension, whereas for an empty tractor with a medium front axle load, they set a low annular chamber pressure and thus preload the suspension to a lower extent and therefore make it softer. Suspension systems with variable annular chamber pressure thus enable improved driving comfort. Such suspension systems are described in U.S. Pat. No. 6,578,855 B2. In this publication, it is proposed to control the annular chamber pressure in inverse proportion to the piston chamber pressure so that the higher the piston chamber pressure, the lower is the annular chamber pressure. At very high piston chamber pressures, the annular chamber pressure is therefore very low. This affects adversely not only the suspension properties of the suspension system but also the hydraulic accumulator connected to the annular chamber.
In DE 197 19 076 A1, a control arrangement of the aforementioned kind is disclosed. In this control arrangement, the annular chamber is connected to the reservoir of the hydropneumatic suspension system via a pressure-limiting line, and provided in the pressure-limiting line is a three-way pressure control valve, the control spring of which is supported on a positioning piston. The positioning piston is configured as a stepped piston that is displaceable in a housing. On the one side, said piston can be acted upon by the spring force of a positioning spring, and on the other side, it can be acted upon by a control pressure that corresponds to the pressure prevailing in the piston chamber. By means of an adjustable end stop, the movability of the positioning piston is limited. Such a configuration of the control arrangement offers the possibility to control the annular chamber pressure in inverse proportion to the piston chamber pressure only at relatively low piston chamber pressures, whereas at high piston chamber pressures, the annular chamber pressure assumes a constant value, since under such pressure conditions, the positioning piston comes into engagement on the end stop and therefore, the annular chamber pressure cannot be further reduced by the pressure control valve. Such a behavior is of advantage for the suspension properties of the suspension system. However, disadvantageous are the high component costs and the complex control as well as the vulnerability to failure of such control arrangements for which no standard components can be used, so that producing such control arrangements is associated with significant costs.
It is therefore an object of the present invention to provide a control arrangement and a hydropneumatic suspension system comprising a control arrangement of this kind which enable variable annular chamber pressure adjustment with as little effort, complexity and production costs as possible while using standard components.